(473d) Thermodynamics of Bacteria to Determine Maximum Bioethanol Production

In a recent paper we have shown how one can calculate the Enthalpy and Gibbs Free Energy of formation of living cells. [Griffiths et al]. Once we have these values we are in principle in a position to treat biological processes in a similar way to normal chemical processes. [Patel et al, 2007]

As more emphasis is placed on sustainable process biological processes will become more important. Understanding how these processes work and what can be achieved from them will become valuable and necessary in plant design. These biological processes are inherently complicated, however they are not excluded from the laws of thermodynamics. We can treat the biological process like a typical chemical process. Using the thermodynamic properties of bacteria the heat (Enthalpy) and work (Gibbs Free Energy) flows of the process can be analysed.

By treating the system as a black box it can be simplified and the main equations can be set up. From the equations a feasible product spectrum can be calculated for the desired products. This data can be used to determine the maximum selectivity of the biological process and the by products that will be produced with it. The maximum selectivities can be used to evaluate the biological process's efficiency. The byproducts can help determine what impact the bioreactor would have on a large scale plant. In this paper we will look at an example of bioethanol production.

As the oil price gradually increases there is a greater push towards renewable fuels such as bioethanol. Bacteria used to produce bioethanol typically follow the Embden-Meyerhof pathway. The other products from this pathway are lactate and acetate. An important aspect of this process is how much ethanol we can produce relative to the lactate and acetate. This is the target of the research. The use of this target will show what side products must be made and their limits.

Further considerations are how this target depends on bacteria species, reactor design and process design. Using an overall systems approach, a thermodynamic analysis can be performed on the bacterial process. This analysis incorporates mass flow, heat flow (Enthalpy) and work flow (Gibbs Free Energy). This simple yet powerful investigation allows us to calculate the thermodynamic limit of ethanol production. The theoretical predictions are compared with data published in literature. Although this approach is illustrated on a bacterial process it can in principle be applied to any microorganisms such as yeast or fungus. This approach provides a powerful process synthesis tool that can be used to evaluate microbial process efficiency and process economics using minimum data.

Griffiths, C., Darko, G., Hildebrandt, D. & Glasser, D.., Thermodynamic Analysis of a Microbial Cell: A Process Synthesis Approach. Paper in preparation

Patel, B., Hildebrandt, D., Glasser, D. & Hausberger, B. Synthesis and Integration of Chemical Processes from a Mass, Energy, and Entropy Perspective. Ind. Eng. Chem. Res. 46, 8756-8766 (2007).